1. Field of the Invention
The present invention relates to a magnetic element.
2. Background of the Invention
Conventionally, many magnetic elements having a structure in which a rectangular or cylindrical ring core is disposed around the periphery of a circular drum core, in which a coil is wound around a winding axis, are known (see, for example, Japanese patent laid-open publication 2006-73847). However, in the magnetic elements having the structure described above, there is a problem that the ends of the coil being wound around the winding axis of the drum core are difficult to be pulled out toward the terminals when connecting the terminals with the coil because the ring core surrounds the periphery of the drum core.
As a solution to this problem, a configuration is disclosed in Japanese patent laid-open publication 2004-111754 in which a planar core is disposed in four directions consisting of both sides of the axial direction of the winding axis as well as both sides of the perpendicular direction to the winding axis so as to sandwich the coil wound around the columnar core, the directions perpendicular to the four directions in which the planar core described above is provided are opened, and the ends of the coil are drawn out from these opened locations.
FIGS. 11A-11C show an exploded perspective view of a magnetic element 500 of the Japanese patent laid-open publication 2004-111754. The magnetic element 500 comprises an upper first core 501, a lower second core 502, and two coils 503, 504.
The first core 501, shown in FIG. 11(A), comprises a flat plane portion 501a; three planar side legs, 501b, 501b, and 501b, which project from a pair of opposed short ends as well as from the middle of the flat plane portion 501a; and columnar central legs 501d, 501d projecting from the centers of each of the recessed portions 501c, 501c, which are surrounded by the adjacent side legs 501b, 501b. In addition, four openings, 501e, 501e, 501e, 501e, are provided in a pair of opposed long ends along which no side leg 501b is provided.
Each of the two coils 503, 504 shown in FIG. 11(B) is an edgewise coil that is formed by winding rectangular wires coated with insulation. The insulation is peeled back from the beginnings and the ends of the windings of the coils 503, 504, and the ends solder plated and furthermore deformed into L-shaped forms so as to form ends 503a, 504a that are the terminals to be electrically connected.
The second coil 502 shown in FIG. 11C has a rectangular, flat plane shape having short and long sides of lengths substantially identical to those of the short and long sides of the first core 501.
The coils 503, 504 fit into the recessed portions 501c, 501c of the first core 501, in a state in which the central legs 501d, 501d are inserted into center openings 503b, 504b. Then, in a state in which the coils 503, 504 are inserted into the recessed portions 501c, 501c of the first core 501, the second core 502 and the first core 501 are brought together, and the recessed portions 501c, 501c are sealed by the second core 502.
Therefore, on both sides in the winding axis direction of the coils 503, 504, the flat plane portion 501a of the first core 501 and the second core 502 are disposed. In addition, in directions perpendicular to the winding axis of coil 503, side legs 501b, 501b are disposed so as to sandwich the coil 503, and moreover, in directions perpendicular to the winding axis of coil 504, side legs 501b, 501b are disposed so as to sandwich the coil 504. In other words, in the four directions of the coil 503, a closed magnetic path is formed by the flat plane portion 501a of the first core 501, the second core 502, the side legs 501b and 501b. In addition, in the four directions of the coil 504, a closed magnetic path is formed by the flat plane portion 501a of the first core 501, the second core 502, the side legs 501b and 501b. 
By contrast, in the recessed portion 501c in which the coil 503 is holded, the openings 501e and 501e are formed. In addition, in the recessed portion 501c in which the coil 504 is holded, the openings 501e and 501e are formed.
As a result, from these openings 501e, 501e, 501e and 501e, the ends of the coils 503 and 504 can be drawn out easily.
However, with the magnetic element having the structure disclosed in Japanese Patent Laid-open publication 2004-111754, because the side legs 501b, 501b, 501b are planar, their cross-sectional area is small and magnetic saturation is easily caused.
If the thicknesses of the side legs 501b, 501b, 501b are increased and their cross-sectional area is increased, then in order not to increase the mounting surface area of the magnetic element 500, it is necessary to increase the thicknesses of the side legs 501b, 501b, 501b toward the side of the coils 503, 504. When that is done, distance between the side legs 501b, 501b, 501b and the central legs 501d, 501d becomes narrower. As a result, the number of windings of the coils 503 and 504 is limited, and it is impossible to increase inductance value sufficiently. In addition, as such distance becomes narrower, when an attempt is made to increase the number of windings of the coils 503, 504, it is necessary to reduce the thicknesses of the winding wires, then it becomes impossible to achieve direct current resistance reduction. Conversely, if increasing the thicknesses of the side legs 501b, 501b, 501b toward the opposite side of the coils 503, 504, the size of the magnetic element 500 itself increases.